My problem is that in SharePoint Online I have a list with a choice column. With "ex1, ex2, ex3" as default chocies, users are allowed to add own choices (fill-in is set to yes).

In the modern UI when you expand the drop down list you first get "ex1, ex2, ex3" and then whatever options users has added. Is there a way to permanently remove these alternative choices (due to spelling errors etc)?

well the problem is that they still want to be able to add new fill-in choices. But the solution (to atleast get rid of the ones with spelling errors) is to choose other choices where the incorrect one is selected. Seems that the drop down looks at whatever custom choice each list item might have and ads them to the list.
– MattiasOct 18 '17 at 12:19